...the life of the planet began the long, slow process of modulating and regulating the physical conditions of the planet. The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms.

About two-thirds of the oxygen in our atmosphere is produced in the surface waters of the sea by phytoplankton, the minute forms of algae that give the sea its slightly green hue, and which initiate the entire food web of the ocean.

Being the inventor of sex would seem to be a sufficient distinction for a creature just barely large enough to be seen by the naked eye.
[Comment about Volvox, a freshwater green algae, which appears indetermimately plantlike and animal-like during its reproductive cycle.]

I … object to dividing the study of living processes into botany, zoology, and microbiology because by any such arrangement, the interrelations within the biological community get lost. Corals cannot be studied without reference to the algae that live with them; flowering plants without the insects that pollinate them; grasslands without the grazing mammals.

I shall never forget the sight. The vessel of crystallization was three quarters full of slightly muddy water—that is, dilute water-glass—and from the sandy bottom there strove upwards a grotesque little landscape of variously colored growths: a confused vegetation of blue, green, and brown shoots which reminded one of algae, mushrooms, attached polyps, also moss, then mussels, fruit pods, little trees or twigs from trees, here, and there of limbs. It was the most remarkable sight I ever saw, and remarkable not so much for its profoundly melancholy nature. For when Father Leverkühn asked us what we thought of it and we timidly answered him that they might be plants: “No,” he replied, “they are not, they only act that way. But do not think the less of them. Precisely because they do, because they try as hard as they can, they are worthy of all respect.”
It turned out that these growths were entirely unorganic in their origin; they existed by virtue of chemicals from the apothecary's shop.

In the beginning was the book of Nature. For eon after eon, the pages of the book turned with no human to read them. No eye wondered at the ignition of the sun, the coagulation of the earth, the birth of the moon, the solidification of a terrestrial continent, or the filling of the seas. Yet when the first primitive algae evolved to float on the waters of this ocean, a promise was born—a hope that someday all the richness and variety of the phenomena of the universe would be read with appreciative eyes.

The entire range of living matter on Earth from whales to viruses and from oaks to algae could be regarded as constituting a single living entity capable of maintaining the Earth’s atmosphere to suit its overall needs and endowed with faculties and powers far beyond those of its constituent parts.